Ah, you're talking about the "Cargo Organizer". It used to be included with the 4R TE, but is now a $85 accessory Toyota p/n PT924-89100-20. :whipping2
I bought one thinking that it might be a good thing, but soon realized that it is a bit useless. Especially if you have the "Cargo Tray" Toyota p/n PT218-89110. This keeps stuff from rolling off in any direction and is nice rubber, which keeps stuff from sliding. The organizer and tray are not compatible with each other.